Co-created by Steven Moffat and Mark Gatiss, Sherlock stars Benedict Cumberbatch as the new Sherlock Holmes and Martin Freeman as his loyal friend, Doctor John Watson. Rupert Graves plays Inspector Lestrade.
The iconic details from Conan Doyle's original books remain – they live at the same address of 221b Baker Street, have the same names and, somewhere out there, Moriarty is waiting for them.

Steven Moffat says: "Conan Doyle's stories were never about frock coats and gas light; they're about brilliant detection, dreadful villains and blood-curdling crimes – and frankly, to hell with the crinoline. Other detectives have cases, Sherlock Holmes has adventures, and that's what matters."
This three part series starts on 25 July.

All of the main people involved are busy doing other things, so I think they just fit in filming whenever they can.

And I guess it's not as uncommon in the UK to have weird season formats/lengths. They have avoided a lot of the "we're doing 20+ episodes a season until no one watches and we cancel it" thing that plagued US television for so long, and still does a bit.